1. Field of the Invention
This invention relates to a recording machine that transfers an electrostatic image onto transfer material such as paper to form a visible image on it, and more particularly to an electrostatographic apparatus that forms a visible image by electrophotography or ion-deposition techniques.
2. Description of the Related Art
Apparatuses that form an electrostatic image on a recording medium, develops it, and transfers the resulting image onto transfer material such as paper to form a color image, are roughly divided into electrophotography systems and ion-deposition systems (such as Japan Hardcopy 89, NIP-6), both now in practical use.
Color image producing apparatuses based on the electrophotography technique are generally of the following two types:
(1) One type of system that forms a toner image for each color on a photosensitive element, transfers it onto recording paper, and superimposes each toner image on one another on transfer paper to form a color image.
(2) The other type of system that superimposes toner images of various colors on each other on a photosensitive element and forms a color image on recording paper through one transfer.
The former type of system (1) that superimposes toner images of various colors one on top of another on transfer paper to form a color image will be explained, referring to FIG. 31 showing the prior art.
In FIG. 31, a photosensitive element 1701 made up of an organic photoconductor (OPC: organic photo convector) is negatively charged by a corona charger 1702 and scanned by the laser beam 1701 modulated by the Y (yellow) picture signal, using a rotary mirror 1703. The electrostatic image of the Y picture formed on the photosensitive element 1701 by the laser beam scanning is developed by means of a Y development apparatus 1706 installed in a developing unit 1705, using a Y (yellow) toner. This developing unit 1705, which is composed of, for example, the Y (yellow), M (magenta), C (cyan), and B (black) development apparatuses 1706 through 1709, is able to perform each color development by changing the development apparatus by rotation. The toner image on the photosensitive element 1701 is transferred onto a transfer sheet fixed on a transfer drum 1710 rotating in synchronism with the photosensitive element 1701, using a transfer corona charger 1711. The transfer sheet is fed in the direction of arrow 1713 from a transfer sheet stoker 1712 so that the leading edge of the toner image may coincide with that of the transfer sheet in synchronization with the picture signal, and then is secured to the transfer drum 1710. After the transfer of the toner image, the residual toner on the photosensitive element 1701 is removed by a wiping-off unit 1714 for reuse.
In this way, the Y, M, C, and B color toner images are superimposed on one another on the transfer sheet passing over the transfer drum 1710. Immediately after this, the transfer sheet is separated from the surface of the transfer drum 1710, fed in the direction of arrow 1715, and fixed by a heat fixing unit 1716, with the result that a fixed color picture is formed on the transfer sheet. As described above, the transfer sheet is fastened to the surface of the transfer drum 1710 to produce a drift-free color picture (print) on the transfer sheet.
In the conventional color image forming apparatus described above, the transfer drum 1710 must be larger than the maximum size (or width) of the transfer sheet to accommodate it. This apparatus needs a plurality of color development apparatus, three or four of which must be provided between the exposure and transfer processes on the recording drum 1701. Changing the plurality of processors while rotating them results in the large, complicated mechanism of the developing unit.
On the other hand, in the later type of system that superimposes color toner images on each other on the photoconductor element to form a color image, and transfers the color toner image to a plane paper by one pass color printing process (cf. the Electrophotography Society, Vol. 28, No. 3, 1989, p.40), the thickness (for example, as thick as one layer of normal toner) of each color toner layer is such that illumination can reach the photosensitive element for charging and illumination to form a color image from the toner image on the photosensitive element. This makes half-tone imaging difficult, which limits applications to multicolor print output.
In the system of this type, the recording drum must be larger in width than the size of the recording image as in the electrophotography technique.
A color imaging printer using electro-static force has been proposed which uses a solid-state corona ionflow head for high-speed control of corona ion flow for each dots and forms a color picture by a single turn of the recording drum (as disclosed in Published Unexamined Patent Application No. 60-237466). With this apparatus, first an electrostatic image is formed using a solid-state corona ion head, and developed by a development apparatus having color toner. After this, the potential of the recording drum on which the color toner image is formed is removed by a discharging corona charger. The image producing stage, which is performed by the solidstate corona ion head, development apparatus, and discharging corona charger, and others, is the process of superimposing color toner images by color one on top of another in sequence using as many kinds of toners as colors required, the toners being prepared on the periphery of the drum (as disclosed in detail in Published Unexamined Japanese Patent Application No. 61-184562).
Because the above-mentioned solid-state ion-flow head provides control of dense ions, using this type of head allows high-speed recording faster than that by laser printers. As the high electro-static contrast of image increases during its formation, this causes the ion beam to bend, and then the pixels start to spread at an electrostatic contrast of approximately 100 V. For the pixels without spreading, an electrostatic contrast of approximately 150 V is the maximum. An attempt to achieve a high electrostatic contrast in the voltage range of 350 V to 500 V for two-component development degrades the resolution of pixel. In a development using magnetic toner that enables development in a low electrostatic contrast, the color of magnetic material in the toner makes color development impossible. The solid state ion head is not suitable for compact design because it forms a color toner image on the drum using ion development or commonly used two-component development. Heat fixing of the color toner image transferred onto the recording sheet requires a fixing unit with a high heat capacity and a large power consumption, which means a long time required for the fixing unit to get ready for use. Accordingly, the user has to wait for a long time from when he turns on the unit until it is ready for use. The heat, which raises the temperature of the recording drum, can degrade its properties. Mechanically removing the residual toner fused on the drum requires metal blades for cleaning. Therefore, the recording drum must be an expensive inorganic insulating drum with high heat resistance and high surface smoothness such as aluminum.
In conventional electrophotography and ion-deposition techniques, the double-side recording that forms the tone images on both sides of the recording sheet needs a mechanically complicated reversing feed mechanism for recording sheets and the technique of feeding a sheet from the same recording paper feeder and recording on both sides of the sheet by the same recording process. Because of the complexity of the feeding mechanism, the application of double-side recording is limited only to monochromatic recording apparatuses. Fixing to double-side-recorded sheet smears the feeding roller of the heat fixing unit due to the already formed toner image, which makes it impossible to reuse the heat fixing roller.
Recording machines based on electrophotography techniques feature less noise because of nonimpact recording devices, legible printing, high-speed recording, and relatively low running cost. Therefore, they are now widely used as the output terminal devices of office automation equipment and their market is rapidly expanding.
For the electrophotographic recording machines, not only laser printers but also light-emitting diodes that serve as recording heads for the writing of electrostatic images, tend to be used and some of them have been developed for commercialization. Laser printers are based on the principle of scanning a light beam generated from a laser by means of a polygonal mirror mechanically rotating at a high speed and a hologram. With the recent trend toward compact design and low cost, solid state scanning systems using an array light source are now attracting more and more attention. For example, there are electrophotographic recording apparatuses already developed and put to practical use, which use a head formed by arranging optical shutters or light-emitting elements such as LEDs, liquid-crystal shutters, EL elements, plasma light-emitting elements, and fluorescent dots. These electrophotographic recording machines are generally called photographic printer using optical device and have found their application to output devices such as printers and digital copiers.
There is another recording system called Ion-Deposition imaging, where insulation layer is used instead of photosensitive elements, and ions are sprayed on the insulation layer from an array of small holes to record an electrostatic image. Those electrophotographic recording machines explained earlier are similar to each other in that recording is carried out through each of the following steps: charging, latent image formation, development, transfer, and fixing.
In general, electrostatic recording machines are characterized by a very small amount of energy required for formation of electrostatic images. Simple comparison of energy values shows that electrophotographic recording machines are far more efficient and much less power consuming than heat-transfer recording machines. Actually, however, electrophotographic recording machines consume power equal to or more than heat-transfer recording machines. In the recording process in the electrophotographic recording machine, the processes from the charging to the transfer of a toner image onto paper are achieved using a very small amount of energy. The final process of fixing toner onto the recording sheet, however, consumes a large amount of energy, which increases the overall power consumption of the electrophotographic recording machine.
Most electrophotographic recording machines today perform fixing with heat (i.e. pressure fixing) and pressure by means of heat rolls. Fixing units using heat rolls are safe because of no danger of combustion. The large heat capacity makes it possible to always provide a stabilized picture quality. In comparison with pressure fixing in the fixing process, the fixing quality is acceptable. The most serious drawback is that the large heat-capacity heat roll needs a warmup time of several minutes because the temperature of the heat roll takes much time to reach the temperature necessary for fixing, making it impossible to start the unit immediately after the switch has been turned on. To increase the heat capacity of the heat roll requires a heater that consumes much power. Because conventional electrophotographic recording machines use heat rolls with a large heat capacity as fixing units, they spend a large amount of power and need a long warmup time. For compact design of these apparatuses, it is undesirable to use heat rolls with a large power consumption and a large heat dissipation.
The processes of transferring the developed toner image onto the recording sheet are handled on the drum in a single stage, while the fixing process is done in a separate stage. Because fixing energy is very large, these two stages are necessary. For compactness, however, use of two stages is not desirable.
Accordingly, color recording machines based on conventional electrophotographic or ion-deposition techniques have the following problems to solve:
Problem 1: First, in the color recording machine using conventional electrophotographic or ion-deposition systems, the technique of superimposing color toner images on each other on the recording drum requires the drum to be larger than the recording image. To achieve this, the diameter of the drum must be made large and the apparatus must be constructed so that the development apparatus may be switched in the order of necessary colors by rotating or sliding them, resulting in an increase in the size. In addition to this, a plurality of color development apparatus must be provided on the periphery of the recording drum between the electrostatic image forming stage and the transfer stage. In consequence, the color recording machine becomes complex in its construction and its recording drum gets larger, which makes it difficult to make the size of the apparatus smaller.
Problem 2: A common drawback to those color recording machines is: when a waste toner pack storing waste toner caused in the cleaning process gets full, the user has to dispose of the waste toner. Because the amount of waste toner actually created in color recording is generally several times as much as that in monochrome recording, it is quite a burden on the side of the user.
Problem 3: In the conventional electrophotographic system, to perform double-side recording of monochromatic pictures, after the recording sheet on one side of which an image has been recorded is mechanically reversed once, it is then sent back to the original feeder inlet. After this, the same recording process is repeated to form an image, resulting in the complicated recording sheet feeding mechanism.
The fusion of the already recorded toner image on the recording sheet in the fixing process smears the fixing roller considerably. This shortcoming is another cause of impairing the maintenance of the apparatus.
Problem 4: In the fixing unit using a heat roll, because of the large heat capacity of the heat roll, it takes a long time for the temperature of the heat roll to reach the temperature necessary for fixing, that is, the unit requires a warmup time. Because the roll has a large heat capacity, it needs a heater that produces a large amount of heat, leading to a large power consumption.